In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters typically include inductors and capacitors, and more recently resonators.
As will be appreciated, it is desirable to reduce the size of many components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials and the thicknesses of the layers in the acoustic stack. One type of BAW resonator comprises a piezoelectric film for the piezoelectric material. These resonators are often referred to as Film Bulk Acoustic Resonators (FBAR).
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of microns and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to certain known resonators.
FBARs may comprise a membrane (also referred to as the acoustic stack) disposed over air. Often, such a structure comprises the membrane suspended over a cavity provided in a substrate over which the membrane is suspended. In other FBARs the acoustic stack is disposed over an acoustic mirror formed in the substrate. Regardless of whether the acoustic stack is suspended over air or provided over an acoustic mirror, the acoustic stack comprises a piezoelectric layer disposed over a first electrode, and a second electrode disposed over the piezoelectric layer.
Filters based on FBAR technology provide a comparatively low in-band insertion loss due to the comparatively high quality (Q) factor of FBAR devices. FBAR-based filters are often employed in cellular or mobile telephones that can operate in multiple frequency bands. In such devices, it is important that a filter intended to pass one particular frequency band (“the passband”) should have a high level of attenuation at other nearby frequency bands which contain signals that should be rejected. Specifically, there may be one or more frequencies or frequency bands near the passband which contain signals at relatively high amplitudes that should be rejected by the filter. In such cases, it would be beneficial to be able to increase the filter's rejection characteristics at those particular frequencies or frequency bands, even if the rejection at other frequencies or frequency bands does not receive the same level of rejection.
One type of filter based on FBAR technology is known as a coupled resonator filter (CRF). A CRF comprises a coupling structure disposed between two vertically stacked FBARs. The CRF combines the acoustic action of the two FBARs, which leads to a bandpass filter transfer function. For a given acoustic stack, the CRF has two fundamental resonance modes, a symmetric mode and an asymmetric mode, of different frequencies. The degree of difference in the frequencies of the modes depends, inter alia, on the degree or strength of the coupling between the two FBARs of the CRF. If the degree of coupling between the two FBARs is too great, the passband is unacceptably wide, and an unacceptable ‘swag’ or ‘dip’ in the center of the passband results, as does an attendant unacceptably high insertion loss in the center of the passband.
The spreading of the passband, and the swag in the center of the passband, has lead efforts to reduce the degree of coupling between the FBARs of the CRF. For many known materials useful for acoustic coupling, the degree of coupling is too great, and results in an unacceptably high difference in the resonance frequencies of the modes of the CRF. One technique used to reduce the degree of coupling between the FBARs of the CRF involves the use a coupling structure comprising a plurality of coupling layers with alternating acoustic impedance. At each interface between each coupling layer a partial reflection of the acoustic mode occurs. The multiple interfaces provide an additive effect, and the degree of coupling between the FBARs is beneficially reduced. While coupling structures comprising a plurality of coupling layers facilitate decoupling of the FBARs in the CRF, their fabrication adds complexity to the fabrication process, and ultimately the cost of the resultant product.
Another technique aimed at reducing the degree of coupling between the FBARs of the CRF involves the use of certain comparatively low acoustic impedance materials, such as silicon low-k (SiLK) resin, which is known to one of ordinary skill in the art. While the use of these known low acoustic impedance materials shows promise from the perspective of reduced coupling between FBARs in the CRF, and thereby improved passband characteristics, such known materials exhibit an unacceptably high acoustic attenuation resulting in an unacceptable degree of acoustic loss, and an undesirable reduction in Q.
What is needed, therefore, is a CRF and method of fabrication that overcomes at least the known shortcomings described above.